18  Unit 4: The Past and the Future 5E

Have humans experienced dramatic climate change in the past? How have human populations been impacted by climate change?

HS-ESS2-4 HS-ESS3-1 HS-ESS3-5

19 Investigative Phenomenon

19.1 🕰️ Climate Change Through Human History

When climate change has occurred in the past, human populations have been impacted. The current climate change event is predicted to affect populations in the future as well.

19.1.1 Driving Questions:

• How has climate change affected people in the past?
• Why is climate change a big deal?
• What happened in the warming event after the last glacial maximum?
• What is AMOC and why does it matter?

20 Engage: The Last Glacial Maximum

20.1 Ice Age Earth: 20,000 Years Ago

Plot = require("@observablehq/plot")

// Temperature transition from LGM to Holocene
lgmData = [
  {kya: 25, temp: -8, event: "Last Glacial Maximum"},
  {kya: 22, temp: -9, event: "LGM Peak"},
  {kya: 20, temp: -8, event: "LGM"},
  {kya: 18, temp: -6, event: "Deglaciation begins"},
  {kya: 16, temp: -4, event: "Warming"},
  {kya: 14.7, temp: -1, event: "Bølling-Allerød warming"},
  {kya: 13, temp: 0, event: "Warm period"},
  {kya: 12.9, temp: -6, event: "YOUNGER DRYAS BEGINS"},
  {kya: 12, temp: -5, event: "Younger Dryas"},
  {kya: 11.7, temp: 0, event: "YOUNGER DRYAS ENDS"},
  {kya: 10, temp: 1, event: "Holocene begins"},
  {kya: 8, temp: 0.5, event: "Holocene"},
  {kya: 6, temp: 0.5, event: "Holocene Optimum"},
  {kya: 4, temp: 0, event: "Late Holocene"},
  {kya: 2, temp: -0.3, event: "Pre-industrial"},
  {kya: 0, temp: 1, event: "Today"}
]

viewof showYD = Inputs.toggle({label: "Highlight Younger Dryas Event", value: true})

Plot.plot({
  title: "Temperature Change: Last Glacial Maximum to Present",
  subtitle: "Greenland ice core data (GISP2)",
  width: 800,
  height: 450,
  x: {label: "Thousands of Years Ago", reverse: true, domain: [25, 0]},
  y: {label: "Temperature Anomaly (°C)", domain: [-10, 3]},
  marks: [
    Plot.ruleY([0], {stroke: "#999", strokeDasharray: "4,4"}),
    showYD ? Plot.rect([{x1: 12.9, x2: 11.7, y1: -10, y2: 3}], {
      x1: "x1", x2: "x2", y1: "y1", y2: "y2", 
      fill: "#ffcdd2", fillOpacity: 0.5
    }) : null,
    showYD ? Plot.text([{x: 12.3, y: 2}], {
      x: "x", y: "y", 
      text: d => "YOUNGER DRYAS\nRapid Cooling Event", 
      fill: "#c62828", 
      fontSize: 11, 
      fontWeight: "bold",
      lineAnchor: "top"
    }) : null,
    Plot.line(lgmData, {x: "kya", y: "temp", stroke: "#2196F3", strokeWidth: 2}),
    Plot.dot(lgmData, {x: "kya", y: "temp", fill: d => d.event.includes("YOUNGER") ? "#c62828" : "#1565c0", r: 5})
  ].filter(d => d !== null)
})

20.1.1 🤔 Initial Observations

1. What dramatic event occurred around 12,900 years ago?
2. How fast did temperatures drop during the Younger Dryas?
3. How long did this cold period last?
4. What might have caused such a rapid climate shift?

21 Explore Part 1: The AMOC (Atlantic Meridional Overturning Circulation)

21.1 What is AMOC?

The Atlantic Meridional Overturning Circulation (AMOC) is a massive system of ocean currents that acts like a global conveyor belt, moving warm water from the tropics to the North Atlantic and cold water back southward at depth.

Ocean Conveyor Belt Diagram

The global thermohaline circulation (ocean conveyor belt)

21.2 Interactive AMOC Simulation

viewof salinityLevel = Inputs.range([30, 38], {
  step: 0.5,
  value: 35,
  label: "North Atlantic Salinity (ppt):"
})

viewof freshwaterInput = Inputs.range([0, 100], {
  step: 10,
  value: 0,
  label: "Freshwater Input (ice melt, %):"
})

effectiveSalinity = salinityLevel - (freshwaterInput * 0.05)
amocStrength = Math.max(0, Math.min(100, (effectiveSalinity - 30) * 12.5))
sinkingRate = amocStrength > 50 ? "Normal" : amocStrength > 25 ? "Reduced" : "Minimal"

html`
<div style="display: flex; gap: 20px; flex-wrap: wrap; margin: 20px 0;">
  <div style="flex: 2; min-width: 400px;">
    <svg width="500" height="300" style="background: linear-gradient(180deg, #e3f2fd 0%, #0d47a1 100%); border-radius: 10px;">
      <text x="50" y="50" fill="#333" font-size="12">Equator (Warm)</text>
      <text x="350" y="50" fill="#333" font-size="12">North Atlantic</text>
      
      <path d="M 80 80 Q 250 60 420 100" fill="none" stroke="#e74c3c" stroke-width="${amocStrength/10 + 2}"/>
      <text x="200" y="55" fill="#e74c3c" font-size="11">Warm Surface Current →</text>
      
      <circle cx="420" cy="150" r="${amocStrength/3 + 10}" fill="#1565c0" opacity="0.7"/>
      <text x="420" y="155" fill="white" font-size="10" text-anchor="middle">Sinking</text>
      
      <path d="M 420 200 Q 250 220 80 200" fill="none" stroke="#2196F3" stroke-width="${amocStrength/10 + 2}"
            stroke-dasharray="${amocStrength < 30 ? '10,5' : 'none'}"/>
      <text x="200" y="245" fill="#2196F3" font-size="11">← Cold Deep Current</text>
      
      ${freshwaterInput > 0 ? `
        <circle cx="380" cy="30" r="${freshwaterInput/5 + 5}" fill="#81d4fa" opacity="0.8"/>
        <text x="380" y="35" fill="#01579b" font-size="10" text-anchor="middle">🧊</text>
      ` : ''}
    </svg>
  </div>
  <div style="flex: 1; min-width: 200px; padding: 20px; background: #f5f5f5; border-radius: 10px;">
    <h4 style="margin-top: 0;">AMOC Status</h4>
    <p><strong>Effective Salinity:</strong> ${effectiveSalinity.toFixed(1)} ppt</p>
    <p><strong>AMOC Strength:</strong> ${amocStrength.toFixed(0)}%</p>
    <p><strong>Sinking Rate:</strong> ${sinkingRate}</p>
    <div style="margin-top: 15px; padding: 10px; background: ${amocStrength < 30 ? '#ffcdd2' : amocStrength < 60 ? '#fff9c4' : '#c8e6c9'}; border-radius: 5px;">
      <strong>Status:</strong> ${amocStrength < 30 ? '⚠️ AMOC severely weakened!' : amocStrength < 60 ? '⚡ AMOC weakening' : '✅ AMOC functioning normally'}
    </div>
  </div>
</div>
`

21.3 🌊 How AMOC Works

The key driver of AMOC is DENSITY:

1. Warm, salty water flows north from the tropics
2. In the North Atlantic, water cools and becomes denser
3. High salinity makes the water even denser
4. Dense water sinks to the deep ocean
5. Deep water flows southward along the ocean floor
6. This creates a continuous circulation pattern

What happens when ice melts: - Melting ice sheets add freshwater to the North Atlantic - Freshwater is less dense than saltwater - Less dense water doesn’t sink as well - AMOC slows down or stops

21.4 🔬 Lab Activity: Density-Driven Currents

21.4.1 Materials:

• Clear rectangular container (aquarium or large baking dish)
• Ice cubes (some colored with blue food dye)
• Salt
• Warm water (colored with red food dye)
• Room temperature water
• Thermometer

21.4.2 Procedure:

Part 1: Setting Up the “Ocean” 1. Fill the container with room temperature water 2. Place blue ice cubes at one end (representing the Arctic) 3. Carefully add warm red water at the other end (representing the tropics) 4. Observe the movement of water

Part 2: Testing Salinity Effects 1. Reset the container with fresh water 2. Add salt to the warm water (make it as salty as seawater: ~35g per liter) 3. Add blue ice cubes at the cold end 4. Observe how salinity affects the circulation

Part 3: Simulating Ice Sheet Melt 1. While circulation is established, add plain (fresh) ice water at the “Arctic” end 2. Observe what happens to the circulation pattern 3. Record your observations

21.4.3 Data Table:

Trial	Setup	Circulation Speed	Pattern Observed
1	Basic (no salt)
2	With salt
3	Salt + freshwater input

21.4.4 Analysis Questions:

1. How did adding salt affect the circulation?
2. What happened when you added freshwater to the salty system?
3. How does this relate to what might happen in the real Atlantic Ocean?

22 Explore Part 2: The Younger Dryas Event

22.1 What Caused the Younger Dryas?

22.2 📜 Case Study: The Younger Dryas (12,900 - 11,700 years ago)

The Scenario: - Earth was warming after the Last Glacial Maximum - Massive ice sheets over North America were melting - Lake Agassiz (larger than all Great Lakes combined) formed from meltwater

The Trigger: - Around 12,900 years ago, Lake Agassiz catastrophically drained - Enormous amounts of freshwater flooded into the North Atlantic - This freshwater was less dense than seawater

The Result: - AMOC slowed dramatically or stopped - Heat transfer to Northern Europe ceased - Temperatures in Greenland dropped ~10°C in just decades - The Northern Hemisphere plunged back into ice age conditions - The cold period lasted ~1,200 years

ydDetailData = [
  {year: 13500, temp: -32},
  {year: 13200, temp: -33},
  {year: 13000, temp: -35},
  {year: 12900, temp: -43},
  {year: 12700, temp: -44},
  {year: 12500, temp: -43},
  {year: 12200, temp: -44},
  {year: 12000, temp: -43},
  {year: 11800, temp: -42},
  {year: 11700, temp: -33},
  {year: 11500, temp: -32},
  {year: 11200, temp: -31}
]

Plot.plot({
  title: "Younger Dryas: Greenland Temperature Record",
  subtitle: "Years before present (ice core data)",
  width: 700,
  height: 400,
  x: {label: "Years Before Present", reverse: true},
  y: {label: "Temperature (°C)", domain: [-50, -25]},
  marks: [
    Plot.rect([{x1: 12900, x2: 11700}], {x1: "x1", x2: "x2", y1: -50, y2: -25, fill: "#e3f2fd", fillOpacity: 0.5}),
    Plot.line(ydDetailData, {x: "year", y: "temp", stroke: "#1565c0", strokeWidth: 3}),
    Plot.dot(ydDetailData, {x: "year", y: "temp", fill: "#0d47a1", r: 5}),
    Plot.text([{x: 12300, y: -28}], {x: "x", y: "y", text: d => "Younger Dryas Cold Period", fill: "#1565c0", fontSize: 12, fontWeight: "bold"})
  ]
})

22.2.1 ⚠️ Human Consequences of the Younger Dryas

Agricultural Revolution Delayed: - Warming had allowed early agriculture to begin in the Fertile Crescent - The Younger Dryas ended this experiment - Humans had to return to hunting and gathering

Population Impacts: - Evidence of population decline in Europe - Megafauna extinctions accelerated - Human settlements abandoned in many regions

After the Younger Dryas: - When warming resumed ~11,700 years ago, agriculture began again - This time it succeeded → the Neolithic Revolution - Human civilization as we know it developed

23 Explain: The AMOC-Climate Connection

23.1 🔄 The Feedback Mechanism

Step 1: Ice sheets melt → freshwater enters North Atlantic

Step 2: Freshwater reduces salinity → water less dense

Step 3: Water doesn’t sink as efficiently → AMOC weakens

Step 4: Less heat transported northward → Northern Hemisphere cools

Step 5: BUT… Southern Hemisphere warms (heat accumulates at equator)

Step 6: Warmer Southern Ocean releases more CO₂ → amplifies global effects

23.2 Current AMOC Status

amocHistoryData = [
  {year: 1950, strength: 18},
  {year: 1960, strength: 17.5},
  {year: 1970, strength: 17.8},
  {year: 1980, strength: 17.2},
  {year: 1990, strength: 16.8},
  {year: 2000, strength: 16.5},
  {year: 2005, strength: 17.0},
  {year: 2010, strength: 15.5},
  {year: 2015, strength: 15.0},
  {year: 2020, strength: 14.5},
  {year: 2024, strength: 14.2}
]

Plot.plot({
  title: "Estimated AMOC Strength Over Time",
  subtitle: "Sverdrups (millions of cubic meters per second)",
  width: 700,
  height: 350,
  x: {label: "Year"},
  y: {label: "AMOC Strength (Sv)", domain: [10, 20]},
  marks: [
    Plot.line(amocHistoryData, {x: "year", y: "strength", stroke: "#e74c3c", strokeWidth: 3}),
    Plot.dot(amocHistoryData, {x: "year", y: "strength", fill: "#c62828", r: 5}),
    Plot.linearRegressionY(amocHistoryData, {x: "year", y: "strength", stroke: "#999", strokeDasharray: "5,5"}),
    Plot.text([{x: 1970, y: 12}], {x: "x", y: "y", text: d => "Declining trend", fill: "#999", fontSize: 11})
  ]
})

23.2.1 ✅ Check Your Understanding

1. What drives the sinking of water in the North Atlantic?
2. How does freshwater from melting ice affect AMOC?
3. Why would a weakening AMOC cause Europe to cool even as global temperatures rise?
4. What evidence from the Younger Dryas suggests AMOC can change rapidly?

24 Elaborate: Climate Change Impacts Today and Tomorrow

24.1 Current Ice Sheet Status

iceLossData = [
  {year: 2002, greenland: 0, antarctica: 0},
  {year: 2004, greenland: -100, antarctica: -50},
  {year: 2006, greenland: -250, antarctica: -100},
  {year: 2008, greenland: -500, antarctica: -200},
  {year: 2010, greenland: -800, antarctica: -350},
  {year: 2012, greenland: -1200, antarctica: -500},
  {year: 2014, greenland: -1600, antarctica: -700},
  {year: 2016, greenland: -2000, antarctica: -1000},
  {year: 2018, greenland: -2500, antarctica: -1300},
  {year: 2020, greenland: -3000, antarctica: -1600},
  {year: 2022, greenland: -3500, antarctica: -2000},
  {year: 2024, greenland: -4000, antarctica: -2400}
]

Plot.plot({
  title: "Cumulative Ice Mass Loss from Ice Sheets",
  subtitle: "Gigatonnes (billions of metric tons)",
  width: 700,
  height: 400,
  x: {label: "Year"},
  y: {label: "Ice Mass Change (Gt)"},
  marks: [
    Plot.ruleY([0], {stroke: "#999"}),
    Plot.line(iceLossData, {x: "year", y: "greenland", stroke: "#2196F3", strokeWidth: 3}),
    Plot.line(iceLossData, {x: "year", y: "antarctica", stroke: "#81d4fa", strokeWidth: 3}),
    Plot.text([{x: 2008, y: -800}], {x: "x", y: "y", text: d => "Greenland", fill: "#2196F3", fontSize: 12}),
    Plot.text([{x: 2008, y: -200}], {x: "x", y: "y", text: d => "Antarctica", fill: "#81d4fa", fontSize: 12})
  ]
})

24.2 Sea Level Rise Projections

viewof scenario = Inputs.radio(["Low Emissions (SSP1-2.6)", "Moderate (SSP2-4.5)", "High Emissions (SSP5-8.5)"], {
  label: "Emissions Scenario:",
  value: "Moderate (SSP2-4.5)"
})

slrData = {
  const base = [
    {year: 2020, level: 0},
    {year: 2030, level: 0.08},
    {year: 2040, level: 0.16},
    {year: 2050, level: 0.25}
  ];
  
  if (scenario === "Low Emissions (SSP1-2.6)") {
    return [...base, 
      {year: 2060, level: 0.32},
      {year: 2070, level: 0.38},
      {year: 2080, level: 0.43},
      {year: 2090, level: 0.47},
      {year: 2100, level: 0.50}
    ];
  } else if (scenario === "Moderate (SSP2-4.5)") {
    return [...base,
      {year: 2060, level: 0.35},
      {year: 2070, level: 0.45},
      {year: 2080, level: 0.55},
      {year: 2090, level: 0.65},
      {year: 2100, level: 0.75}
    ];
  } else {
    return [...base,
      {year: 2060, level: 0.40},
      {year: 2070, level: 0.55},
      {year: 2080, level: 0.75},
      {year: 2090, level: 0.95},
      {year: 2100, level: 1.20}
    ];
  }
}

Plot.plot({
  title: `Sea Level Rise Projection: ${scenario}`,
  subtitle: "Meters above 2020 level",
  width: 700,
  height: 400,
  x: {label: "Year", domain: [2020, 2100]},
  y: {label: "Sea Level Rise (m)", domain: [0, 1.5]},
  marks: [
    Plot.areaY(slrData, {x: "year", y: "level", fill: "#0277bd", fillOpacity: 0.3}),
    Plot.line(slrData, {x: "year", y: "level", stroke: "#01579b", strokeWidth: 3}),
    Plot.dot(slrData, {x: "year", y: "level", fill: "#01579b", r: 5}),
    Plot.ruleY([0.5], {stroke: "#e74c3c", strokeDasharray: "5,5"}),
    Plot.text([{x: 2040, y: 0.55}], {x: "x", y: "y", text: d => "0.5m: Major coastal flooding", fill: "#e74c3c", fontSize: 10})
  ]
})

24.3 Populations at Risk

riskData = [
  {region: "Bangladesh", population: 20, type: "Flooding"},
  {region: "Vietnam", population: 12, type: "Flooding"},
  {region: "China (coastal)", population: 50, type: "Flooding"},
  {region: "India (coastal)", population: 15, type: "Flooding"},
  {region: "Indonesia", population: 10, type: "Flooding"},
  {region: "Small Island Nations", population: 5, type: "Complete displacement"},
  {region: "US Gulf Coast", population: 8, type: "Flooding"},
  {region: "Netherlands", population: 4, type: "Flooding"}
]

Plot.plot({
  title: "Populations at Risk from Sea Level Rise",
  subtitle: "Millions of people potentially displaced by 1m rise",
  width: 700,
  height: 400,
  x: {label: "People at Risk (millions)"},
  y: {label: "Region", domain: riskData.map(d => d.region)},
  marks: [
    Plot.barX(riskData, {x: "population", y: "region", fill: d => d.type === "Complete displacement" ? "#c62828" : "#1565c0", sort: {y: "-x"}}),
    Plot.text(riskData, {x: "population", y: "region", text: d => d.population + "M", dx: 20, fontSize: 11})
  ]
})

24.4 Climate Impacts Already Happening

24.4.1 🌍 Current Climate Change Impacts

🌊 Sea Level Rise

Current: ~3.7 mm/year (accelerating)

Total since 1900: ~20 cm

Impact: Coastal erosion, flooding, saltwater intrusion

🔥 Extreme Heat

Trend: Heat waves 5x more likely

Record: 2023 hottest year on record

Impact: Heat deaths, crop failures, wildfires

🌀 Extreme Weather

Trend: More intense hurricanes

Rainfall: Heavier precipitation events

Impact: Flooding, infrastructure damage

🧊 Ice Loss

Arctic: -13% per decade

Glaciers: Retreating worldwide

Impact: Sea level rise, ecosystem loss

🌾 Food Security

Crops: Yields declining in some regions

Fisheries: Species shifting poleward

Impact: Food shortages, price increases

💧 Water Resources

Drought: More frequent and severe

Snowpack: Declining in mountains

Impact: Water shortages, conflicts

24.5 NOAA Sea Level Rise Viewer

Explore how sea level rise will affect coastal communities:

NOAA Sea Level Rise Viewer

Explore interactive maps showing flooding scenarios at different sea levels:

🗺️ Open NOAA Sea Level Rise Viewer

Opens in new tab - explore your community's flood risk

24.6 🔬 Lab Activity: Community Impact Assessment

24.6.1 Task: Investigate Climate Impacts in Your Region

Part 1: Sea Level Rise (if coastal) 1. Go to the NOAA Sea Level Rise Viewer (link above) 2. Find your community or nearest coastal city 3. Toggle between 1ft, 3ft, and 6ft sea level rise scenarios 4. Document: What areas flood? What infrastructure is at risk?

Part 2: Temperature and Precipitation Changes 1. Visit the NOAA Climate Explorer 2. Enter your zip code or city 3. Examine projected changes in: - Number of days above 95°F - Annual precipitation patterns - Number of dry days

Part 3: Analysis Questions 1. What climate impacts are most relevant to your community? 2. Who in your community would be most affected? 3. What adaptations might be needed? 4. How does this compare to what populations experienced during the Younger Dryas?

24.6.2 Data Recording Table:

Climate Variable	Current	2050 Projection	2100 Projection	Potential Impact
Hot days (>95°F)
Sea level (if coastal)
Annual precipitation
Drought frequency

25 Evaluate: Connecting Past to Future

25.1 📊 Synthesizing Your Learning

You now have the knowledge to explain:

25.1.1 1. How Climate Has Changed in the Past

• Milankovitch cycles drove glacial-interglacial cycles
• Rapid changes like the Younger Dryas show climate can shift quickly
• AMOC played a crucial role in past climate events

25.1.2 2. How Humans Were Impacted

• The Younger Dryas delayed the agricultural revolution
• Climate shifts caused population movements and extinctions
• Societies that couldn’t adapt faced collapse

25.1.3 3. What’s Happening Now

• Ice sheets are melting at alarming rates
• AMOC shows signs of weakening
• Populations are already being displaced

25.1.4 4. What May Happen in the Future

• Sea levels will continue to rise
• Extreme weather will intensify
• Millions may be displaced

25.1.5 💡 Key Ideas: The Past and the Future

1. AMOC is slowing as ice sheets melt, similar to what triggered the Younger Dryas
2. Freshwater reduces salinity at the poles, preventing deep water formation
3. Historical AMOC shutdowns caused rapid cooling in the Northern Hemisphere
4. The Younger Dryas had major implications for human populations, delaying civilization
5. Ice sheets are melting at alarming rates today
6. People globally are already being impacted by climate change
7. Learning from the past helps us prepare for and potentially prevent future disasters

---

25.2 📝 The Past and the Future Quiz

Question 1: What is AMOC? - A) A type of greenhouse gas - B) The Atlantic Meridional Overturning Circulation - a major ocean current system - C) A glacier in Antarctica - D) A climate modeling computer

Question 2: What drives the sinking of water in the North Atlantic that powers AMOC? - A) Wind patterns - B) The Moon’s gravity - C) Cold, salty water being denser than warm, fresh water - D) Volcanic activity

Question 3: What triggered the Younger Dryas cooling event? - A) A volcanic eruption - B) A meteor impact - C) Massive freshwater release from melting ice sheets into the North Atlantic - D) Changes in the Sun’s output

Question 4: How long did the Younger Dryas last? - A) About 100 years - B) About 1,200 years - C) About 10,000 years - D) About 1 million years

Question 5: How did the Younger Dryas affect human populations? - A) It had no effect on humans - B) It helped humans develop agriculture faster - C) It delayed the agricultural revolution and caused population decline - D) It caused humans to migrate to Antarctica

Question 6: Current observations show that AMOC is: - A) Strengthening rapidly - B) Staying constant - C) Showing signs of weakening - D) Completely stopped

Question 7: Which ice sheet is losing mass fastest? - A) Greenland - B) Antarctica - C) They’re losing mass at equal rates - D) Neither is losing mass

Question 8: Approximately how many people globally could be displaced by 1 meter of sea level rise? - A) About 1 million - B) About 10 million - C) About 100 million or more - D) About 1,000 people

Question 9: Why is understanding past climate change important for preparing for future climate change? - A) It’s not important - the past is different from the future - B) Past events show how quickly climate can change and how populations can be affected - C) Past climates were always stable - D) We can’t learn anything from ice cores

Question 10: If AMOC were to significantly weaken or collapse today, what would likely happen to Europe? - A) It would get much warmer - B) It would experience significant cooling despite global warming - C) Nothing would change - D) It would experience more earthquakes

Answers: 1-B, 2-C, 3-C, 4-B, 5-C, 6-C, 7-A, 8-C, 9-B, 10-B